<?php

$mysql = new mysqli('localhost', 'root', '', 'test');

$tags = 'my tag, природа, пейзаж, лес';
$photoName = $mysql->real_escape_string('photo1');

$photoIdRaw = $mysql->query('SELECT id FROM photo WHERE photo_name = "' . $photoName . '"')
    ->fetch_row();

if (empty($photoIdRaw)) {
    throw new Exception('No photo by that name: ' . $photoName);
}

$photoId = (int)$photoIdRaw[0];
$insertQueryValues = '';

foreach (explode(',', $tags) as $value) {
    $insertQueryValues .= '("' . $mysql->real_escape_string(trim($value)) . '"),';
}

$mysql->query('CREATE TEMPORARY TABLE tTags (title varchar(255))');

$mysql->query('INSERT INTO tTags VALUES '. rtrim($insertQueryValues, ','));

$mysql->query(
    'INSERT INTO tags (title)
         SELECT tTags.title
         FROM tTags
         LEFT JOIN tags ON 
             tTags.title = tags.title
         WHERE tags.title IS NULL'
);

$mysql->query(
    'INSERT INTO photo_tags (photo_id, tag_id)
         SELECT ' . $photoId . ', tags.id 
         FROM tTags
         INNER JOIN tags ON 
             tTags.title = tags.title
         LEFT JOIN photo_tags ON
             tags.id = photo_tags.tag_id AND
             photo_tags.photo_id = ' . $photoId . '
         WHERE photo_tags.tag_id IS NULL'
);

$mysql->close();